Bio-impedance sensor and sensing method

ABSTRACT

Implantable medical devices and techniques are implemented that use bio-impedance to measure aspects of patient physiology. A signal separation method is performed at least in part in an implantable device. The method involves detecting a plurality of impedance signals using a plurality of implantable electrodes coupled to the implantable device. The method further involves separating one or more signals from the plurality of impedance signals using a signal separation technique, such as an algorithm-based separation technique.

FIELD OF THE INVENTION

The present invention relates generally to implantable medical devicesand, more particularly, to implanted devices and techniques that usebio-impedance to measure aspects of patient physiology.

BACKGROUND OF THE INVENTION

Heart failure is an abnormality of cardiac function that causes cardiacoutput to fall below a level adequate to meet the metabolic demand ofperipheral tissues. Heart failure is often referred to as congestiveheart failure (CHF) due to the accompanying venous and pulmonarycongestion. Heart failure may have a variety of underlying causes,including ischemic heart disease (coronary artery disease), hypertension(high blood pressure), and diabetes, among others. It has been observedthat respiratory disruption can be particularly serious for patientsconcurrently suffering from cardiovascular deficiencies, such as heartfailure. Unfortunately, disordered breathing is often undiagnosed. Ifleft untreated, the effects of disordered breathing may result inserious health consequences for the patient.

Because of the need for early evaluation of heart failure and/ordisordered breathing symptoms, an effective approach to monitoring andearly diagnosis is desired. Acquiring accurate physiological sensorinformation may allow for early intervention, preventing serious heartfailure decompensation and hospitalization.

SUMMARY OF THE INVENTION

The present invention is directed to implantable medical devices andtechniques that use bio-impedance to measure aspects of patientphysiology. In accordance with an embodiment of the present invention, asignal separation method is performed at least in part in an implantabledevice. The method involves detecting a plurality of impedance signalsusing a plurality of implantable electrodes coupled to the implantabledevice. The method further involves separating one or more signalsaccording to their sources from the plurality of impedance signals usinga signal separation technique, such as an algorithm-based separationtechnique.

Methods of the present invention may involve storing the separatedsignal(s) within the implantable device. Methods may involve using theseparated one or more signals within the implantable device for avariety of purposes. The separated signal(s) from the implantable devicemay be transmitted to a patient-external location or device, such as aportable or bed-side communications device or an interface device of anetworked patient management system.

Various signal separation techniques may be implemented. For example,one suitable signal separation technique involves forming a linearcombination of impedance signals that increases a signal-to-noise ratiofor the separated one or more signals. Forming the linear combinationmay involve forming the linear combination of impedance signals usingblind source separation. Another suitable signal separation techniqueinvolves an adaptive noise cancellation technique.

Methods of the present invention may further involve identifying the oneor more separated signals with one or more signals of interest. Oneapproach involves identifying one or more separated signals by themaximum of the correlation between the separated signal(s) and thesignal of interest. The one or more signals of interest may comprise aphysiological signal. For example, the physiological signal of interestmay be a signal associated with respiration or cardiac activity. Thephysiological signal may be a signal useful for detecting presence ofthoracic fluid.

Identifying the one or more separated signals may involve matching amorphology of a separated signal with a morphology of a signal ofinterest. For example, identifying the one or more separated signals mayinvolve matching frequency content of a separated signal with frequencycontent of a signal of interest. Identifying the one or more separatedsignals may involve matching timing of events within a separated signalwith timing of events within a signal of interest.

In accordance with another embodiment, an implantable medical device maybe configured to include a plurality of implantable electrodes eachconfigured for sensing an impedance signal, thereby providing aplurality of impedance signals. The implantable medical device mayinclude a processor coupled to the plurality of electrodes andconfigured to receive the plurality of impedance signals. The processormay further be configured to separate one or more signals from theplurality of impedance signals using a signal separation technique.

The processor is typically coupled to memory, and the separated one ormore signals may be stored in the memory. The device preferably includescommunication circuitry coupled to the processor. The communicationcircuitry may be configured to transfer the separated one or moresignals to a patient-external receiver. The patient-external receivermay-be part of a patient-worn or held communications device, a bed-sidedevice, or a communications interface of a networked patient managementsystem.

The processor may be configured to form a linear combination ofimpedance signals that increases a signal-to-noise ratio for theseparated one or more signals. For example, the processor may beconfigured to perform blind source separation when forming the linearcombination of impedance signals. Alternatively, the processor may beconfigured to perform adaptive noise cancellation to separate the one ormore signals.

The processor may be configured to identify the one or more separatedsignals with one or more signals of interest, such as a physiologicalsignal. Physiological signals of interest may include a signalassociated with one or more of respiration, cardiac activity, and asignal useful for detecting presence of thoracic fluid.

The processor may be configured to perform matching of a morphology of aseparated signal with a morphology of a signal of interest. Theprocessor may be configured to perform matching of frequency content ofa separated signal with frequency content of a signal of interest. Theprocessor may be configured to perform matching of timing of eventswithin a separated signal with timing of events within a signal ofinterest.

The above summary of the present invention is not intended to describeeach embodiment or every implementation of the present invention.Advantages and attainments, together with a more complete understandingof the invention, will become apparent and appreciated by referring tothe following detailed description and claims taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of a method useful for separating one or moresignals of interest from a multiplicity of impedance signals inaccordance with an embodiment of the present invention;

FIG. 2 is a flow diagram of a method useful for separating one or moresignals of interest from a multiplicity of impedance signals inaccordance with another embodiment of the present invention;

FIG. 3 is a flow diagram of a method useful for separating one or moresignals of interest from a multiplicity of impedance signals inaccordance with a further embodiment of the present invention;

FIG. 4 is a flow diagram of a method useful for separating one or moresignals of interest from a multiplicity of impedance signals inaccordance with another further embodiment of the present invention;

FIG. 5 is a block diagram of circuitry configured to separate one ormore signals of interest from a multiplicity of impedance signals inaccordance with an embodiment of the present invention;

FIG. 6 shows an implantable system suitable for implementing a signalseparation methodology in accordance with an embodiment of the presentinvention;

FIG. 7 shows an embodiment of an implantable medical device suitable forperforming signal separation methodologies in accordance with thepresent invention; and

FIG. 8 shows another embodiment of an implantable medical devicesuitable for performing signal separation methodologies in accordancewith the present invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail below. It is to be understood,however, that the intention is not. to limit the invention to theparticular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings, which form a part hereof, and inwhich is shown by way of illustration, various embodiments in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural and functional changes maybe made without departing from the scope of the present invention.

An implanted device according to the present invention may include oneor more of the features, structures, methods, or combinations thereofdescribed hereinbelow. For example, a cardiac monitor or a cardiacstimulator may be implemented to include one or more of the advantageousfeatures and/or processes described below. It is intended that such amonitor, stimulator, or other implanted or partially implanted deviceneed not include all of the features described herein, but may beimplemented to include selected features that provide for uniquestructures and/or functionality. Such a device may be implemented toprovide a variety of therapeutic or diagnostic functions.

Devices implemented in accordance with the present invention aregenerally referred to herein as a patient implantable medical device(PIMD), which may include a housing implanted under the skin in thechest region of a patient. A PIMD may, for example, be implantedsubcutaneously such that all or selected elements of the device arepositioned on the patient's front, back, side, or other body locationssuitable for sensing cardiac activity and delivering cardiac stimulationtherapy. It is understood that elements of the PIMD may be located atseveral different body locations, such as in the chest, abdominal, orsubclavian region, with electrode elements respectively positioned atdifferent regions near, around, in, or on the heart. For example, a PIMDmay include one or more of endocardial leads, epicardial leads,subcutaneous leads, sensor or electrode modules and arrays, and/or otherleads or sensors.

PIMDs, such as cardiac rhythm management devices, of the presentinvention preferably employ lead electrode configurations that areconfigured to measure bio-impedance across multiple vectors, including avariety of intracardiac, trans-cardiac, and trans-thoracic vectors.Impedance measurements across any of these vectors are typicallyinfluenced by multiple aspects of patient physiology, includingrespiratory and cardiac activity. PIMDs and sensing techniques accordingto embodiments of the present invention advantageously provide forseparating out the effects of these individual physiological sourcesignals from the impedance measurement by using multiple impedancemeasurement vectors.

PIMDs and sensing techniques according to embodiments of the presentinvention advantageously provide an optimal linear combination of amultiplicity of impedance vectors to enable higher sensitivity of thecombined signal to a physiologic parameter of interest. Such physiologicparameters of interest may include lung fluid, respiratory components,or cardiac components. By including a local impedance in the linearcombination, the combined signal could be made robust to physiologicnoises, such as hematocrit or electrolyte changes, for example. By wayof further example, intra-cardiac impedance signals may be useful forpatient hemodynamic monitoring and assessment. Cardiac output and volumemay be assessed, as can be filling and/or ejection patterns particularlyuseful for optimizing cardiac resynchronization therapy. PIMDs andsensing techniques may provide for higher specificity and sensitivityfor an advanced generation bio-impedance sensor.

Some implanted devices use transthoracic impedance to monitor minuteventilation. A typical technique involves filtering out the portion ofthe impedance signal caused by the cardiac contraction or “stroke.” Thisapproach can be successful if the heartbeat frequency is consistentlyhigher than the frequency of respiration. This conventional approach hasbeen used successfully for years to measure minute ventilation for rateresponsive pacing.

Recently, research into using this same respiration sensing technologyto provide diagnostic measures of respiration in heart failure patientshas identified limitations in such conventional approaches. In certainsituations, for example, the heart rate may not always be consistentlyhigher than the respiratory rate. In other situations, heart rate may betoo unstable (e.g., atrial fibrillation) to effectively remove thecardiac stroke component.

In these cases, the cardiac stroke component cannot be effectivelyfiltered out without also affecting the respiratory signal that one isattempting to measure. It has been observed that traditional signalprocessing techniques based on measurements along a single impedancevector are not always successful in removing cardiac stroke componentsfrom transthoracic impedance waveforms. These limitations may notsignificantly affect the performance of rate responsive pacing, but havebeen shown to lead to highly erroneous diagnostic measures ofrespiration. The present inability to successfully remove orsignificantly attenuate the cardiac stroke component under variedphysiologic conditions can lead to falsely elevated estimates ofrespiration. Moreover, patient hemodynamic monitoring usingsingle-vector impedance measurements have not proven successful.

PIMDs and sensing techniques according to the present invention areimplemented to overcome the limitations of conventional approaches suchas, for example, by attenuating the cardiac stroke component of animpedance signal to improve the accuracy of respiration sensing.According to embodiments of the invention, known signal processingalgorithms employed for adaptive noise cancellation in communicationschannels (e.g., echo cancellation in telephone systems) may beadvantageously exploited and adapted for use in the context of thepresent invention. An adaptive noise cancellation approach of thepresent invention removes components correlated to a measured noisesource from a composite signal (i.e., signal of interest plus noise).

In other embodiments, an algorithm may be employed to separate signalcomponents originating from different sources, e.g., separating cardiacstroke and respiratory signals from low signal-to-noise (SNR) ratiosignals using a separation technique, such as blind source separation.According to one approach, a PIMD is configured to identify an optimalcombination of the impedance vector signals that makes cardiac andrespiration components, as well as noise, independent (i.e., orthogonal)to one another. This technique relies upon the principle that signalsarising from a common source, when sensed by spatially distributedelectrodes, will be correlated strongly in time and in space along aparticular direction.

According to an embodiment that employs a source separation approach,respiratory and cardiac components can be isolated by revealing thecorrelated spatial components with a time-averaged cross-correlationmatrix. Such an approach further includes projecting signals on anorthonormal basis, which makes the components from the different sourcesorthogonal (i.e., uncorrelated) to one another. A separation techniquesuch as blind source separation, for example, uses statistical signalprocessing to enhance spatial correlation and find these optimaldirections (i.e., linear combination coefficients).

It is known that respiratory disturbances are a hallmark of heartfailure decompensation. Respiratory diagnostics can play a central rolein decompensation prediction and detection, particularly in the contextof an advanced patient management system as described herein.Respiration sensing, however, must be sufficiently accurate to enablereliable respiratory diagnostic trending. Cardiac stroke componentsleaking into the respiratory waveform, for example, have beendemonstrated to corrupt respiratory diagnostic measures.

Devices and methods implemented in accordance with the present inventionenable more reliable respiration sensing, even in cases of unstableheart rate, and in cases of overlapping heart and respiratory rates.Devices and methods of the present invention can be implemented toprovide useful hemodynamic measures via joint processing of multiplecardiac impedance vectors. Devices and methods can be implemented toprovide for closed loop cardiac resynchronization therapy optimizationby use of cardiac impedance parameters in accordance with the presentinvention.

Although aspects of the present invention focus on removing cardiacstroke from transthoracic impedance, it is understood that the presentinvention has applicability to the more general problem of separatingmultiple physiological signals from impedance measurements alongmultiple vectors.

Sensing and/or stimulation devices that separate individual impedancesignals from multiple sensed impedance signals in accordance with thepresent invention may be adapted to their implant environment manually,such as by a clinician after implantation, or may be adapted toautomatically configure themselves. Electrode arrays and/or multipleelectrodes provide for many possible combinations useful for sensingimpedance, respiratory activity, cardiac activity, patient activity, andother signals useful for evaluating patient well-being and treatingadverse patient conditions, such as cardiac arrhythmia.

Turning now to the figures, FIGS. 1-4 illustrate various methodologiesdirected to separating one or more signals from a multiplicity ofimpedance signals using a algorithmic signal separation technique. Ingeneral terms, embodiments of the present invention are directed to theuse of impedance from various vectors to separate out various sources.For example, devices and methods of the present invention preferablyinvolve jointly or concurrently processing impedance signals frommultiple vectors for a wide variety of purposes. Useful vectors includeintracardiac, trans-cardiac, and trans-thoracic vectors.

FIG. 1 is a flow diagram of a method useful for separating one or moresignals of interest from a multiplicity of impedance signals inaccordance with an embodiment of the present invention. According toFIG. 1, two or more impedance signals are detected 102, typically usinga multiplicity of implantable electrodes. One or more signals areseparated 104 from the detected impedance signals using an algorithmicseparation technique. According to one approach, signal separation 104may involve an adaptive noise cancellation technique. According toanother approach, signal separation may involve a source separationtechnique, such as blind source separation.

FIG. 2 is a flow diagram of a method useful for separating one or moresignals of interest from a multiplicity of impedance signals inaccordance with another embodiment of the present invention. Accordingto FIG. 2, two or more impedance signals are detected 202, and one ormore signals are separated 204 from the detected impedance signals usingan algorithmic separation technique. The method of FIG. 2 furtherinvolves identifying 206 the separated signal(s) with one or moresignals of interest. One approach involves identifying one or moreseparated signals by the maximum of the correlation between theseparated signal(s) and the signal of interest. A physiological signalrepresents one such signal of particular interest. The signal ofinterest may be a signal associated with respiration, a signalassociated with cardiac activity, or a signal associated with thoracicfluid, for example.

FIG. 3 is a flow diagram of a method useful for separating one or moresignals of interest from a multiplicity of impedance signals inaccordance with a further embodiment of the present invention. Accordingto FIG. 3, two or more impedance signals are detected 302, one or moresignals are separated 304 from the detected impedance signals using analgorithmic separation technique, and one or more of the separatedsignals are identified 306 with one or more signals of interest.According to FIG. 3, identifying 306 the separated signal(s) with one ormore signals of interest may involve matching 308 a morphology of aseparated signal with a morphology of a signal of interest.

Morphology matching may involve comparison of morphological features,such as fiducial points, inflection points, minima, maxima, or otherfeatures, for example, and may involve computing a correlationcoefficient on a feature-by-feature basis, for example. Patternrecognition and pattern matching techniques may also be used to performthe morphology matching.

For example, various known pattern and/or feature recognition techniquesmay be employed, such as by using neural networks and other statisticalpattern recognition techniques. Such techniques may include principalcomponent analysis, fisher and variance weight calculations and featureselection. Neural network methods may include a back propagation neuralnetwork and/or radial basis function neural network. Statistical patternrecognition may include linear discriminant analysis, quadraticdiscriminant analysis, regularized discriminant analysis, softindependent modeling of class analogy, and/or discriminant analysis withshrunken covariance.

Identifying 306 the separated signal(s) with one or more signals ofinterest may involve matching 310 the frequency content of a separatedsignal with the frequency content of a signal of interest. Various knownfrequency content matching techniques may be used, including spectralanalysis techniques. For example, spectral analysis techniques mayinvolve comparing the dominant frequency or frequencies of the signalwith the frequency regions of sources of interest, or may involvecomparing the morphology of the frequency content to the frequencycontent of a signal of interest.

Identifying 306 the separated signal(s) with one or more signals ofinterest may involve matching 312 timing of events within a separatedsignal with the timing of events within a signal of interest. Suchtechniques may include comparing the timings of minima and/or maxima (orother signal feature) in the signal to expected timings of sources ofinterest. For example, in the case of cardiac sources, thesource-of-interest timings may be cardiac intervals, whereas in the caseof respiratory sources, the source-of-interest timings may berespiratory intervals.

FIG. 4 is a flow diagram of a source separation method useful forseparating one or more signals of interest from a multiplicity ofimpedance signals in accordance with another embodiment of the presentinvention. The method shown in FIG. 4 involves obtaining 402 multipleconcurrent measurements between multiple respective electrode pairs,chosen from at least three electrodes, for example. Collected signalsmay be pre-filtered 404 by, for example, a linear-phase filter tosuppress broadly incoherent noise, and to generally maximize thesignal-to-noise ratio.

A cross-correlation matrix may be computed 406, which may be averagedover a relatively short time interval, such as about 1 second.Eigenvalues of the cross-correlation matrix may be computed 408. Thesmaller eigenvalues, normally associated with noise, may be eliminated410, by removing the noise components of the composite signalsassociated with those eigenvalues.

Individual signals may be separated from the composite signals using theeigenvalues. Separated sources may be obtained 412 by taking linearcombinations of the recorded signals, as specified in the eigenvectorscorresponding to the larger eigenvalues. Optionally, additionalseparation may be performed 414 based on higher order statistics, if thedesired signal is not found among the signals separated at block 412.

The impedance signal may be identified 416 based on selection criteria,along with its associated vector, among the separated signals.Typically, the signal may be found among the signals associated with thelargest eigenvalues. The vector associated with the selected signal maythen be used to determine an impedance signal in accordance with thepresent invention.

The methodology illustrated in FIG. 4, such as a blind source separationtechnique, can use information from multiple leads to successfullyrestore a signal of interest even during a time when the signal iscorrupted by noise or signals of other sources. In many cases, it isunlikely that any degree of filtering or signal processing based on onlyone lead would be able to correctly restore the signal of interest inthe presence of corrupting noise. Only with the additional spatialinformation provided by multiple sources can the signal of interest beseparated out successfully. For example, only with multiple impedancevectors can one expect to be able to separate out multiple sources ofimpedance fluctuations that overlap in both time and frequency.

Signal separation methodologies and electrode and vector selectionmethodologies useful in the context of a source separation approach ofthe present invention are further described in commonly owned U.S.patent application Ser. No. 10/876,008 filed Jun. 24, 2004, and U.S.Patent Publication No. 2004/0230128 (Ser. No. 10/741,814, filed Dec. 19,2003), which are hereby incorporated herein by reference.

FIG. 5 is a block diagram of circuitry configured to separate one ormore signals of interest from a multiplicity of impedance signals inaccordance with another embodiment of the present invention. Thecircuitry shown in FIG. 5 includes adaptive noise cancellation circuitry502 that has a primary input 510 and a reference input 512. A signalsource 504 is shown providing an input signal to the primary input 510.The primary input 510 is coupled to a summer 516, and presents a signal,s+n₀ (a composite signal comprising a signal component and a noisecomponent), to the summer 516.

A noise source 506 is shown providing an input signal to the referenceinput 512. The reference input 512 is coupled to an adaptive filter 514,the output 513 of which is coupled to the summer 516. An errorcorrection signal is communicated from the output of the summer 516 tothe adaptive filter via a feedback path 515. An output signal of thenoise cancellation circuitry 502 is provided at an output 520. A PIMD ofthe present invention may incorporate the noise cancellation circuitryshown in FIG. 5 to separate one or more signals of interest from amultiplicity of impedance signals.

In accordance with one application involving the use of the circuitryshown in FIG. 5, the signal source 504 may represent the purelyrespiratory portion of an impedance waveform. It is noted that this purerespiratory signal is not directly measurable in the physical system,and is, in fact, the signal of interest to be estimated from theavailable inputs (e.g., the primary and reference inputs 510, 512 inFIG. 5).

The noise source 506 represents the cardiac stroke component of theimpedance waveforms. A replica of the cardiac stroke component can bemeasured in the physical system by performing an intracardiac impedancemeasurement. This measurement provides the signal to the reference input512 in FIG. 5. The intracardiac impedance waveform is assumed to be areplica of the cardiac stroke component that corrupts the transthoracicimpedance measurement of respiration.

In operation, the adaptive filter 514 self-adjusts its parameters tominimize the difference between the primary input signal (e.g., originalraw minute ventilation impedance waveform) and the filter output 513,thereby subtracting off or effectively canceling all portions of theprimary input signal that are linearly related to the cardiac strokesignal.

Turning now to FIG. 6, an implantable system in accordance with anembodiment of the present invention is shown having a lead systemdeployed within a heart. System 801 includes a PIMD 800 with a leadsystem 802 that is designed for implantation to facilitate cardiacresynchronization therapy. The lead system 802 is coupled to adetection/energy delivery system 900, which detects cardiac activity anddelivers appropriate therapy via the lead system 802.

The detector/energy delivery system 900 typically includes a powersupply and programmable circuit (e.g., microprocessor) coupled to ananalog to digital (A-D) converter. Various lead system devices, such aselectrodes and pressure sensors, can interface to the A-D converter forsensing/data collection. Alternatively, analog conditioning (e.g.,filtering) may be applied to sensor signals before interfacing with theA-D converter. The detector/energy delivery system 900 also utilizes anenergy delivery system. The energy delivery system may include chargecapacitors and signal conditioning circuitry known in the art. Theenergy delivery system may interface to the programmable circuit througha D-A converter. Components and functionality of the detector/energydelivery system 900 will be further described below with reference toFIG. 7.

Still referring to FIG. 6, the PIMD system 801 is shown having the PIMD800 electrically and physically coupled to the lead system 802. Thehousing and/or header of the PIMD 800 may incorporate one or moreelectrodes 908, 909 used to provide electrical stimulation energy to theheart and to sense cardiac electrical activity. The PIMD 800 may utilizeall or a portion of the PIMD housing as a can electrode 909. The PIMD800 may include an indifferent electrode 908 positioned, for example, onthe header or the housing of the PIMD 800. If the PIMD 800 includes botha can electrode 909 and an indifferent electrode 908, the electrodes908, 909 typically are electrically isolated from each other.

The lead system 802 is used to provide pacing signals to the heart 803,detect electric cardiac signals produced by the heart 803, and deliverelectrical energy to the heart 803 under certain predeterminedconditions, such as to improve cardiac output and/or to treat cardiacarrhythmias. The lead system 802 may include one or more electrodes usedfor pacing, sensing, and/or defibrillation. In the embodiment shown inFIG. 6, the lead system 802 includes an intracardiac right ventricular(RV) lead system 804, an intracardiac right atrial (RA) lead system 805,an intracardiac left ventricular (LV) lead system 806, and anextracardiac left atrial (LA) lead system 808. The lead system 802 ofFIG. 6 illustrates one of many possible PIMD configurations. It isunderstood that more or fewer leads and/or electrodes of varying typemay be used.

The right ventricular lead system 804 illustrated in FIG. 6 includes anSVC-coil 816, an RV-coil 814, an RV-ring electrode 811, and an RV-tipelectrode 812. The right ventricular lead system 804 extends through theright atrium 820 and into the right ventricle 819. In particular, theRV-tip electrode 812, RV-ring electrode 811, and RV-coil electrode 814are positioned at appropriate locations within the right ventricle 819for sensing and delivering electrical stimulation pulses to the heart.The SVC-coil 816 is positioned at an appropriate location within theright atrium chamber 820 of the heart 803 or a major vein leading to theright atrial chamber 820 of the heart 803.

In one configuration, the RV-tip electrode 812 referenced to the canelectrode 909 may be used to implement unipolar pacing and/or sensing inthe right ventricle 819. Bipolar pacing and/or sensing in the rightventricle may be implemented using the RV-tip 812 and RV-ring 811electrodes. In yet another configuration, the RV-ring 811 electrode mayoptionally be omitted, and bipolar pacing and/or sensing may beaccomplished using the RV-tip electrode 812 and the RV-coil 814, forexample. The right ventricular lead system 804 may be configured as anintegrated bipolar pace/shock lead. The RV-coil 814 and the SVC-coil 816are defibrillation electrodes.

The left ventricular lead 806 includes an LV distal electrode 813 and anLV proximal electrode 817 located at appropriate locations on thesurface of, or about, the left ventricle 824 for pacing and/or sensingthe left ventricle 824. The left ventricular lead 806 may be guided intothe right atrium 820 of the heart via the superior vena cava. From theright atrium 820, the left ventricular lead 806 may be deployed into thecoronary sinus ostium, the opening of the coronary sinus 850. The lead806 may be guided through the coronary sinus 850 to a coronary vein ofthe left ventricle 824. This vein is used as an access pathway for leadsto reach the surfaces of the left ventricle 824 which are not directlyaccessible from the right side of the heart. Lead placement for the leftventricular lead 806 may be achieved via subclavian vein access and apreformed guiding catheter for insertion of the LV electrodes 813, 817adjacent to the left ventricle.

Unipolar pacing and/or sensing in the left ventricle may be implemented,for example, using the LV distal electrode referenced to the canelectrode 909. The LV distal electrode 813 and the LV proximal electrode817 may be used together as bipolar sense and/or pace electrodes for theleft ventricle. The left ventricular lead 806 and the right ventricularlead 804, in conjunction with the PIMD 800, may be used to providecardiac resynchronization therapy such that the ventricles of the heartare paced substantially simultaneously, or in phased sequence, toprovide enhanced cardiac pumping efficiency in accordance with thepresent invention for patients suffering from heart failure.

The right atrial lead 805 includes a RA-tip electrode 856 and an RA-ringelectrode 854 positioned at appropriate locations in the right atrium820 for sensing and pacing the right atrium 820. In one configuration,the RA-tip 856 referenced to the can electrode 909, for example, may beused to provide unipolar pacing and/or sensing in the right atrium 820.In another configuration, the RA-tip electrode 856 and the RA-ringelectrode 854 may be used to provide bipolar pacing and/or sensing. Thelead system 802 may include one or more extracardiac leads 808 havingelectrodes, e.g., epicardial electrodes or sensors 815, 818, positionedat locations outside the heart for sensing and/or pacing one or moreheart chambers.

Referring now to FIG. 7, there is shown an embodiment of a PIMD 900suitable for performing signal separation methodologies in accordancewith the present invention. FIG. 7 shows the PIMD 900 divided intofunctional blocks. It is understood by those skilled in the art thatthere exist many possible configurations in which these functionalblocks can be arranged. The example depicted in FIG. 7 is one possiblefunctional arrangement. Other arrangements are also possible. Forexample, more, fewer or different functional blocks may be used todescribe a PIMD suitable for performing signal separation methodologiesin accordance with the present invention. In addition, although the PIMD900 depicted in FIG. 7 contemplates the use of a programmablemicroprocessor-based logic circuit, other circuit implementations may beutilized. It is also understood that the components and functionalitydepicted in FIG. 7 and elsewhere may be implemented in hardware,software, or a combination of hardware and software.

The PIMD 900 depicted in FIG. 7 includes circuitry for receiving cardiacsignals from a heart and delivering electrical stimulation energy to theheart in the form of pacing pulses and/or defibrillation shocks. In oneembodiment, the circuitry of the PIMD 900 is encased and hermeticallysealed in a housing 901 suitable for implanting in a human body. Powerto the PIMD 900 is supplied by an electrochemical battery 980. Aconnector block (not shown) is attached to the housing 901 of the PIMD900 to allow for the physical and electrical attachment of the leadsystem conductors to the circuitry of the PIMD 900.

The PIMD 900 may be a programmable microprocessor-based system,including a control system 920 and a memory 970. The memory 970 maystore parameters for various pacing, resynchronization, defibrillation,and sensing modes, along with other parameters. Further, the memory 970may store data indicative of signals received by other components of thePIMD 900. The memory 970 may be used, for example, for storinghistorical information, impedance information, sensor information, bloodflow information, perfusion information, heart sounds, heart movement,EGM information, therapy data, and/or other information, assumingappropriate sensors are provided. The historical data storage mayinclude, for example, data obtained from long-term patient monitoring(e.g., respiratory data) used for trending patient well-being, heartfailure decompensation, or other diagnostic purposes. Historical data,as well as other information, may be transmitted to an external device990 as needed or desired. In one embodiment, the external device 990 mayinclude an communications interface of a networked patient managementsystem, as is discussed below.

The control system 920 and memory 970 may cooperate with othercomponents of the PIMD 900 to control the operations of the PIMD 900.The control system depicted in FIG. 7 incorporates a processor 925 forclassifying cardiac responses to pacing stimulation. The control system920 may include additional functional components including an,arrhythmia detector 921, a pacemaker control circuit 922, and a templateprocessor 923 for cardiac signal morphology analysis, along with othercomponents for controlling the operations of the PIMD 900.

The PIMD 900 may also include a signal processor 924 for performingsignal separation in accordance with the present invention. Signalseparation may be performed using the control system 920 and/or thesignal processor 924 to perform the separation operations by the PIMD900, or the signal separation may be performed in a patient-externaldevice 990 communicatively coupled to the PIMD 900. The PIMD 900 mayinclude noise cancellation circuitry 927 for performing signalseparation in accordance with an adaptive noise cancellation methodologyof the present invention. For example, the noise cancellation circuitry927 may be configured and operate in accordance with the embodimentshown in FIG. 5.

A PIMD 900 implemented in accordance with the present invention may havea control system 920 and communications circuitry 960 that transmits itssignals to a bedside signal processor when the patient is asleep. Thebedside signal processor may perform a blind source separation andanalysis of the signals during the patient's sleep cycle. The signalprocessor may then determine the appropriate impedance sensingconfiguration, and reprogram the PIMD before the patient awakes. ThePIMD may then operate with the latest programming until the next update.

Communications circuitry 960 may be implemented to providecommunications between the PIMD 900 and an external programmer unit 990and/or APM system. In one embodiment, the communications circuitry 960and the programmer unit 990 communicate using a wire loop antenna and aradio frequency telemetric link, as is known in the art, to receive andtransmit signals and data between the programmer unit 990 and thecommunications circuitry 960. In this manner, programming commands andother information may be transferred to the control system 920 of thePIMD 900 from the programmer unit 990 during and after implant.

The communications circuitry 960 may also allow the PIMD to communicatewith one or more receiving devices or systems situated external to thePIMD 900. By way of example, the PIMD may communicate with apatient-worn, portable or bedside communication system via thecommunications circuitry 960. In one configuration, one or morephysiologic or non-physiologic sensors (subcutaneous, cutaneous, orexternal of patient) of a PIMD system may be equipped with a short-rangewireless communication interface, such as an interface conforming to aknown communications standard, such as Bluetooth or IEEE 802 standards.Data acquired by such sensors may be communicated to the PIMD 900 viathe communications circuitry 960. It is noted that physiologic ornon-physiologic sensors equipped with wireless transmitters ortransceivers may communicate with a receiving system external of thepatient. The external sensors in communication with the PIMD 900 may beused to facilitate patient well-being assessment, heart failuredecompensation trending/tracking, cardiac resynchronization therapyadjustment and optimization, and other purposes.

In the embodiment of the PIMD 900 illustrated in FIG. 7, electrodesRA-tip 856, RA-ring 854, RV-tip 812, RV-ring 811, RV-coil 814, SVC-coil816, LV distal electrode 813, LV proximal electrode 817, LA distalelectrode 818, LA proximal electrode 815, indifferent electrode 908, andcan electrode 909 are coupled through a switch matrix 910 to sensingcircuits 931-937.

A right atrial sensing circuit 931 serves to detect and amplifyelectrical signals from the right atrium of the heart. Bipolar sensingin the right atrium may be implemented, for example, by sensing voltagesdeveloped between the RA-tip 856 and the RA-ring 854. Unipolar sensingmay be implemented, for example, by sensing voltages developed betweenthe RA-tip 856 and the can electrode 909. Outputs from the right atrialsensing circuit are coupled to the control system 920.

A right ventricular sensing circuit 932 serves to detect and amplifyelectrical signals from the right ventricle of the heart. The rightventricular sensing circuit 932 may include, for example, a rightventricular rate channel 933 and a right ventricular shock channel 934.Right ventricular cardiac signals sensed through use of the RV-tip 812electrode are right ventricular near-field signals and are denoted RVrate channel signals. A bipolar RV rate channel signal may be sensed asa voltage developed between the RV-tip 812 and the RV-ring 811.Alternatively, bipolar sensing in the right ventricle may be implementedusing the RV-tip electrode 812 and the RV-coil 814. Unipolar ratechannel sensing in the right ventricle may be implemented, for example,by sensing voltages developed between the RV-tip 812 and the canelectrode 909.

Right ventricular cardiac signals sensed through use of the RV-coilelectrode 814 are far-field signals, also referred to as RV morphologyor RV shock channel signals. More particularly, a right ventricularshock channel signal may be detected as a voltage developed between theRV-coil 814 and the SVC-coil 816. A right ventricular shock channelsignal may also be detected as a voltage developed between the RV-coil814 and the can electrode 909. In another configuration, the canelectrode 909 and the SVC-coil electrode 816 may be electrically shortedand a RV shock channel signal may be detected as the voltage developedbetween the RV-coil 814 and the can electrode 909/SVC-coil 816combination.

Left atrial cardiac signals may be sensed through the use of one or moreleft atrial electrodes 815, 818, which may be configured as epicardialelectrodes. A left atrial sensing circuit 935 serves to detect andamplify electrical signals from the left atrium of the heart. Bipolarsensing and/or pacing in the left atrium may be implemented, forexample, using the LA distal electrode 818 and the LA proximal electrode815. Unipolar sensing and/or pacing of the left atrium may beaccomplished, for example, using the LA distal electrode 818 to canvector 909 or the LA proximal electrode 815 to can vector 909.

Referring still to FIG. 7, a left ventricular sensing circuit 936 servesto detect and amplify electrical signals from the left ventricle of theheart. Bipolar sensing in the left ventricle may be implemented, forexample, by sensing voltages developed between the LV distal electrode813 and the LV proximal electrode 817. Unipolar sensing may beimplemented, for example, by sensing voltages developed between the LVdistal electrode 813 or the LV proximal electrode 817 and the canelectrode 909.

Optionally, an LV coil electrode (not shown) may be inserted into thepatient's cardiac vasculature, e.g., the coronary sinus, adjacent to theleft heart. Signals detected using combinations of the LV electrodes,813, 817, LV coil electrode (not shown), and/or can electrodes 909 maybe sensed and amplified by the left ventricular sensing circuitry 936.The output of the left ventricular sensing circuit 936 is coupled to thecontrol system 920.

Turning now to FIG. 8, the primary housing (e.g., the active ornon-active can) of the PIMD 701, for example, may be configured forpositioning outside of the rib cage at an intercostal or subcostallocation, within the abdomen, or in the upper chest region (e.g.,subclavian location, such as above the third rib). In one configuration,as is illustrated in FIG. 8, electrode subsystems of a PIMD system arearranged about a patient's heart 710. The PIMD system includes anelectrode arrangement comprising a can electrode 702 provided on all ora portion of the PIMD housing 703. An optional electrode assembly 704 isalso illustrated in FIG. 8 that may include one or more of electrodes,sensors, and multi-element electrodes. The optional electrode assembly704 is coupled to the PIMD housing 703 using a lead 706.

In various configurations, the optional electrode subsystem 704 mayinclude a combination of electrodes. The combination of electrodes ofthe optional electrode subsystem 704 may include coil electrodes, tipelectrodes, ring electrodes, multi-element coils, spiral coils, spiralcoils mounted on non-conductive backing, screen patch electrodes, andother electrode configurations. A suitable non-conductive backingmaterial is silicone rubber, for example.

In one configuration, the lead assembly 706 is generally flexible andhas a construction similar to conventional implantable, medicalelectrical leads (e.g., defibrillation leads or combineddefibrillation/pacing leads). In another configuration, the leadassembly 706 is constructed to be somewhat flexible, yet has an elastic,spring, or mechanical memory that retains a desired configuration afterbeing shaped or manipulated by a clinician. For example, the leadassembly 706 may incorporate a gooseneck or braid system that may bedistorted under manual force to take on a desired shape. In this manner,the lead assembly 706 may be shape-fit to accommodate the uniqueanatomical configuration of a given patient, and generally retains acustomized shape after implantation. Shaping of the lead assembly 706according to this configuration may occur prior to, and during, PIMDimplantation.

In accordance with a further configuration, the lead assembly 706includes a rigid electrode support assembly, such as a rigid elongatedstructure that positionally stabilizes the electrode 704 with respect tothe PIMD housing 703. In this configuration, the rigidity of theelongated structure maintains a desired spacing between the electrode704 and PIMD housing 703, and a desired orientation of the electrode704/housing 703 relative to the patient's heart. The elongated structuremay be formed from a structural plastic, composite or metallic material,and includes, or is covered by, a biocompatible material. Appropriateelectrical isolation between the PIMD housing 703 and electrode 704 isprovided in cases where the elongated structure is formed from anelectrically conductive material, such as metal.

In one configuration, the rigid electrode support assembly and thehousing 703 define a unitary structure (e.g., a single housing/unit).The electronic components and electrode conductors/connectors aredisposed within or on the unitary PIMD housing/electrode supportassembly. At least two electrodes are supported on the unitary structurenear opposing ends of the housing/electrode support assembly. Theunitary structure may have an arcuate or angled shape, for example.

According to another configuration, the rigid electrode support assemblydefines a physically separable unit relative to the housing 703. Therigid electrode support assembly includes mechanical and electricalcouplings that facilitate mating engagement with correspondingmechanical and electrical couplings of the housing 703. For example, aheader block arrangement may be configured to include both electricaland mechanical couplings that provide for mechanical and electricalconnections between the rigid electrode support assembly and housing703. The header block arrangement may be provided on the housing 703 orthe rigid electrode support assembly. Alternatively, amechanical/electrical coupler may be used to establish mechanical andelectrical connections between the rigid electrode support assembly andhousing 703. In such a configuration, a variety of different electrodesupport assemblies of varying shapes, sizes, and electrodeconfigurations may be made available for physically and electricallyconnecting to a standard PIMD housing 703.

It is noted that the electrode(s) 704 and the lead assembly 706 may beconfigured to assume a variety of shapes. For example, the lead assembly706 may have a wedge, chevron, flattened oval, or a ribbon shape, andthe electrode 704 may include a number of spaced electrodes, such as anarray or band of electrodes. Moreover, two or more electrodes 704 may bemounted to multiple electrode support assemblies 706 to achieve adesired spaced relationship amongst electrodes 704.

A PIMD of the present invention may be used within the structure of anadvanced patient management (APM) medical system. Advanced patientmanagement systems may allow physicians to remotely and automaticallymonitor cardiac and respiratory functions, as well as other patientconditions. In one example, implantable cardiac rhythm managementsystems, such as cardiac pacemakers, defibrillators, andresynchronization devices, may be equipped with varioustelecommunications and information technologies that enable real-timedata collection, diagnosis, and treatment of the patient. Variousembodiments described herein may be used in connection with advancedpatient management. Methods, structures, and/or techniques describedherein, which may be adapted to provide for remote patient/devicemonitoring, trending, diagnosis, therapy, or other APM relatedmethodologies, may incorporate features of one or more of the followingreferences: U.S. Pat. Nos. 6,221,011; 6,270,457; 6,277,072; 6,280,380;6,312,378; 6,336,903; 6,358,203; 6,368,284; 6,398,728; and 6,440,066,which are hereby incorporated herein by reference.

Various embodiments described herein may be used in connection withheart failure monitoring, trending, diagnosis, and/or therapy. A PIMD ofthe present invention may incorporate features involving dual-chamber orbi-ventricular pacing/therapy, cardiac resynchronization therapy,cardiac function optimization, or other heart failure relatedmethodologies. For example, a PIMD of the present invention mayincorporate features of one or more of the following references:commonly owned U.S. patent application Ser. No. 10/270,035, filed Oct.11, 2002, entitled “Timing Cycles for Synchronized Multisite CardiacPacing;” and U.S. Pat. Nos. 6,411,848; 6,285,907; 4,928,688; 6,459,929;5,334,222; 6,026,320; 6,371,922; 6,597,951; 6,424,865; and 6,542,775,each of which is hereby incorporated herein by reference.

A PIMD may be used to implement various diagnostic functions, which mayinvolve performing rate-based, pattern & rate-based, and/ormorphological tachyarrhythmia discrimination analyses. Subcutaneous,cutaneous, and/or external sensors may be employed to acquirephysiologic and non-physiologic information for purposes of enhancingtachyarrhythmia detection and termination. It is understood thatconfigurations, features, and combination of features described in thepresent disclosure may be implemented in a wide range of implantablemedical devices, and that such embodiments and features are not limitedto the particular devices described herein.

Various modifications and additions can be made to the preferredembodiments discussed hereinabove without departing from the scope ofthe present invention. Accordingly, the scope of the present inventionshould not be limited by the particular embodiments described above, butshould be defined only by the claims set forth below and equivalentsthereof.

1. A signal separation method performed at least in part in animplantable device, the method comprising: detecting a plurality ofimpedance signals using a plurality of implantable electrodes coupled tothe implantable device; and separating one or more signals from theplurality of impedance signals using a signal separation technique. 2.The method of claim 1, further comprising storing the separated one ormore signals within the implantable device.
 3. The method of claim 1,further comprising using the separated one or more signals within theimplantable device.
 4. The method of claim 1, further comprisingtransferring the separated one or more signals from the implantabledevice to a patient-external location.
 5. The method of claim 1, whereinthe signal separation technique comprises forming a linear combinationof impedance signals that increases a signal-to-noise ratio for theseparated one or more signals.
 6. The method of claim 5, wherein formingthe linear combination comprises forming the linear combination ofimpedance signals using blind source separation.
 7. The method of claim1, wherein the signal separation technique comprises adaptive noisecancellation.
 8. The method of claim 1, further comprising identifyingthe one or more separated signals with one or more signals of interest.9. The method of claim 8, wherein identifying the one or more separatedsignals comprises identifying the one or more separated signals by amaximum of a correlation between the one or more separated signals and asignal of interest.
 10. The method of claim 8, wherein the one or moresignals of interest comprise a physiological signal.
 11. The method ofclaim 10, wherein the physiological signal comprises a signal associatedwith respiration.
 12. The method of claim 10, wherein the physiologicalsignal comprises a signal associated with cardiac activity.
 13. Themethod of claim 10, wherein the physiological signal comprises a signaluseful for detecting presence of thoracic fluid.
 14. The method of claim8, wherein identifying the one or more separated signals comprisesmatching a morphology of a separated signal with a morphology of asignal of interest.
 15. The method of claim 8, wherein identifying theone or more separated signals comprises matching frequency content of aseparated signal with frequency content of a signal of interest.
 16. Themethod of claim 8, wherein identifying the one or more separated signalscomprises matching timing of events within a separated signal withtiming of events within a signal of interest.
 17. An implantable medicaldevice, comprising: a plurality of implantable electrodes eachconfigured for sensing an impedance signal, thereby providing aplurality of impedance signals; and a processor coupled to the pluralityof electrodes and configured to receive the plurality of impedancesignals, the processor further configured to separate one or moresignals from the plurality of impedance signals using a signalseparation technique.
 18. The device of claim 17, wherein the processoris coupled to memory, and the separated one or more signals are storedin the memory.
 19. The device of claim 17, further comprisingcommunication circuitry coupled to the processor, the communicationcircuitry configured to transfer the separated one or more signals to apatient-external receiver.
 20. The device of claim 17, wherein theprocessor is configured to form a linear combination of impedancesignals that increases a signal-to-noise ratio for the separated one ormore signals.
 21. The device of claim 20, wherein the processor isconfigured to perform blind source separation when forming the linearcombination of impedance signals.
 22. The device of claim 17 wherein theprocessor is configured to perform adaptive noise cancellation toseparate the one or more signals.
 23. The device of claim 17, whereinthe processor is configured to identify the one or more separatedsignals with one or more signals of interest.
 24. The device of claim23, wherein the one or more signals of interest comprise a physiologicalsignal.
 25. The device of claim 24, wherein the physiological signalcomprises a signal associated with one or more of respiration, cardiacactivity, and a signal useful for detecting presence of thoracic fluid.26. The device of claim 23, wherein the processor is configured toperform matching of a morphology of a separated signal with a morphologyof a signal of interest.
 27. The device of claim 23, wherein theprocessor is configured to perform matching of frequency content of aseparated signal with frequency content of a signal of interest.
 28. Thedevice of claim 23, wherein the processor is configured to performmatching of timing of events within a separated signal with timing ofevents within a signal of interest.
 29. An implantable medical device,comprising: means for detecting a plurality of impedance signals using aplurality of implantable electrodes coupled to the implantable device;and means for separating one or more signals from the plurality ofimpedance signals using a signal separation technique.